The temporal variability, hysteresis loops and various factors involved in the mobilization of nitrates (NO 3-) have been studied for a 3-year period at the Flumen River basin. Multivariate techniques (cluster analysis and pearson correlation matrix) were used to establish the relationship between the water discharge and NO 3- flushing, as well as to identify the agricultural and hydrometeorological parameters that influence its different mobilization trends. The relationship between changes in the NO 3- concentration (Delta C) and the overall dynamic of each hysteresis loop (Delta R) was also analyzed in order to describe the NO 3- trends according to the water discharge. A general dilution pattern of the NO 3- concentration was noted in the Flumen River with respect to the degree of water discharge caused by irrigation return flows. While fertilization increased the NO 3- concentration, the beginning of the irrigation season contributed to its dilution. However, in case of the NO 3- load, the maximum values occurred during high flow periods in the irrigation period, which suggested the influence of the irrigation flow on the NO 3- mass. The NO 3- load increased to 2753 t and 1059 t during the first and second phases of the study period, respectively, with an average specific yield of 1.33 t km -2 y -1. The NO 3- transport in the first phase of the study was 1722 t during the irrigation season and 1031 t during the non-irrigation period. Only 348 t (13%) of NO 3- was exported during the flood events. However, in the course of the second phase of the study, the NO 3- load was 733 t during the irrigation season and 326 t during the non-irrigation period. In this case, 610 t (57%) of nitrate was transported during the floods. These results revealed the clear influence of irrigation return flows on the NO 3- response in Flumen River.
